I was in New Mexico at a Japanese restaurant with a couple Spanish-speaking friends. The owner was at our table, talking to them in Spanish. He said something about the date of something, and I corrected him in Japanese. He asked me what I was talking about in Japanese, and I responded to him in Spanish.

He just kind of stared at me for a long second, but then we just spoke in English.

I am not really conversational, per se, in any language but English (and maybe Italian). I have some very specific vocabulary in a couple different languages. But when I am put in certain situations, I speak certain languages automatically. (Japanese restaurants always make me speak Japanese to the server... even in America... even when the server isn't Japanese... I'm too used to the atmosphere.)

When I started taking German in college, when people would talk to me in German, I would respond in Italian before I could really notice my mistake. Took a couple days to break the habit. ("Wie geht's?" Sono stanca. Shit, ich bin müde! Verstehen Sie?!)

So, for me, there is no huge "eureka", it just becomes automatic and you don't really notice until after the fact. There wasn't really a process, one day it just happened without effort.